The present invention relates generally to check valves for passing fluids in one direction but preventing fluid flow in the opposite direction. More particularly, to a flapper check valve for use in fluid handling systems where corrosive products are pumped.
In the past, corrosive fluid handling systems requiring check valves have used ball type check valves. These ball check valves were of standard design and were constructed from metal parts. However, they require that their internal surfaces be coated with a thermoplastic or thermoset material that is impervious to the caustic product flowing in the pipeline. The thermoplastic material was bonded to the internal surfaces of the ball check valve through an expensive injection molding process. It was necessary to coat these surfaces to protect the metal parts of the valve from destruction from the caustic fluids. The ball that was used to check the back flow of the caustic fluids was usually made from tetrafluoroethylene (TFE), a material commonly known by the name TEFLON, a registered trademark of DuPont.
A major disadvantage of these prior-art ball check valves was that Teflon does not have memory, i.e. once the Teflon has been deformed, it tends to stay deformed. To check the backflow through the valve, the check ball is cradled in a seat that surrounds an internal opening that was part of the through passage of the valve. The ball, acting against the seat, formed the seal that prevented any back flow. Unfortunately, the ball often times is checked (driven against the seat by the fluid attempting to flow in the opposite direction) against the seat abruptly. Sometimes this abrupt check causes the seat to produce a ring or indentation in the ball. Eventually, the ball begins to lose its generally spherical shape as a result of the repeated checkings and begins to lose its ability to seal against the seat.
A solution to the problem to the Teflon ball wearing out from repeated checkings is to substitute a metal ball and metal seat made from an exotic and expensive metal alloy such as Monel (an alloy of stainless steel and nickel). These metal balls and seats are not coated with a thermoplatic material and eventually are destroyed by the caustic fluids although they tend to last longer than the Teflon balls.
Another disadvantage of ball check valves is that they must be mounted vertically to work properly. In a vertical orientation, the ball falls into the seat, due to gravity, when flow through the valve is zero. In a horizontal position, the ball moves into the seat only if there is reverse flow and, once a seal is formed, there is sufficient back pressure to hold the ball in the seat.
The disadvantages discussed above of ball type check valve are not present in flapper type check valves. The flapper seals against a flat surface and does not experience the deterioration to the sealing surfaces that the ball in the ball check valve does. Also, flapper check valves may be installed in any orientation.
While flapper type check valves are known in the art, workable designs for flapper check valves for use in fluid handling systems where corrosive fluids are involved have not been developed. Thus, it would be advantageous to provide a flapper type check valve for use in corrosive fluid handling systems where the internal check valve parts are impervious to the corrosive fluids. It would also be advantageous to provide a flapper check valve of either the flange or waffer design that was both inexpensive to manufacture and simple to service. It would also be advantageous to provide a streamlined flapper check valve where all parts of the valve were contained within a cylindrical metal sleeve having no cumbersome exterior bonnets to accommodate the hinged flapper. It would also be advantageous to provide a flapper check valve in which the flapper seals the through passage of the valve at a smaller back pressure than is normally required in valves of that type design without the need for a spring to urge the flapper against the sealing surface when the flow is zero.